At present, ring-type current transformers (“CTs”) are the most prevalent technology for measuring phase currents in three-phase electric power transmission and distribution lines. While very accurate, these devices are quite expensive when applied at high voltages. This results, in large part, from the need to insulate the current sensors for high-voltage, which is quite expensive for voltages above 1,000 volts. There have been other approaches using other types of magnetic field sensors, such as a Hall Effect sensor, located close to each phase conductor. In multiphase applications, this technique is complicated by magnetic field interference from the adjacent phases, which results in inaccurate measurements. As a result, the only successful technique using this type technology has been to use a ring of magnetic field sensors that completely encircles the conductor to balance out the extraneous error signals from other phases. This approach also requires expensive high-voltage insulation for the magnetic field sensors, which makes the system economically feasible only at the highest transmission line voltages.
Another approach to high voltage current measurement uses fiber optic sensors that pass polarized light around each conductor. The magnetic field generated by the phase current rotates the polarization direction in proportion the current. While this technique reduces the difficulty of insulating the current sensor at high voltages, the sophisticated decoding technology required to turn the polarization measurement into a current measurement is quite expensive. Again, this approach is economically feasible only at the highest transmission line voltages.
Another type of power system unknown occurs because high currents and ambient air heat the electric conductors, which causes them to stretch and sag from their overhead supports. If this sag becomes too large, a voltage breakdown and resulting flash over can occur between the power line and another object, such as a tree or hill. It is therefore desirable to monitor the physical sag of the power line at critical locations during normal operating conditions. Algorithms have be used to estimate the physical sag of the power line based on the current values measured at the substation, ambient conditions, and the physical configuration of the line section of interest. But these algorithms only produce estimates of the physical sag at critical locations that can vary significantly from the actual conditions due to voltage drop and reactive power loading on the lined during high load conditions. Another approach has been to take direct distance measurement using optical systems that typically include laser distance finders and cameras. But these optical systems are very expensive, do not work well in the dark, can be adversely affected by bad weather and fouling by grime, bird droppings and the like.
Accordingly, there is an ongoing need for a cost effective electric power monitoring and response systems that avoid insulation problems and costly decoding systems. There is a further need for a electric power line sag monitoring system that does not rely on optical components.